Scientific Forecaster Discussion
fxus64 kmrx 202010
Area forecast discussion
National Weather Service Morristown Tennessee
310 PM EST Wed Nov 20 2019
Short term...(tonight and thursday)
Latest rap analysis shows that surface high pressure has settled
into the lower Ohio Valley with an anticyclonic northerly flow across
the southern Appalachians. This has finally allowed a drier airmass
to advect into the region, and expect the clouds to quickly
dissipate the rest of the afternoon. Mid/upper level shortwave
ridging is beginning to build in from the SW with a warm front
draped across MO and southern Illinois into west TN, so these rising
heights combined with the clearing sky have caused temps to nicely
spike into the mid 50's in many areas the past couple of hours.
The surface high will slide east across the Appalachians tonight
while becoming elongated with mid/upper level heights further
rising. This is in response to a strong shortwave trough ejecting
out of a southwest U.S. Closed low into the upper Midwest. Soundings
show a weak warm air advection pattern developing overnight as the low-level flow
becomes weak southerly. This will hold lows in the mid/upper 30's
across most areas with some staying near 40 in the southern valley.
There will be some lingering low-level moisture, but high level
cirrus moving in from the west and larger than ideal dew point
depressions should prevent fog.
Thursday will be the nicest day in quite some time. The
aforementioned shortwave moving into the upper Midwest will induce
strengthening surface low pressure that will be moving into the
central Great Lakes with its associated cold front reaching the
Mississippi Valley by 00z Fri. SW flow will deepen ahead of this
front through the day pumping a warm airmass into the region. Clouds
will be on the increase, but there will be enough sunshine to push
temps well into the 60's. Went a couple of degrees above guidance
for highs. Brought in slight chance pops for the plateau by 23z, but
most areas will stay dry.
Long term...(thursday night through wednesday)
An active extended period with a series of northern and southern
stream jets moving across the Ohio/Tennessee valleys. The GFS and
European model (ecmwf) are in fairly good agreement with the latest week/weekend
systems, but major differences for the early next week system/
cyclogenesis. Lets get to the particulars.
For late Thursday night through Saturday, a series of jets will
enhance fronto-genetic forcing and isentropic lift across the area.
The strongest of the waves will move through Friday night and
Saturday. Periods of rain showers will move across the region
through this time-frame with locally heavy rainfall possible as pws
increase to above 90th percentile. Will include this concern in the
severe weather potential statement.
For Saturday night and Sunday morning, orographic lift from
northwest flow aloft and wrap-around moisture from exiting system
will keep chances of showers going, especially over the higher
terrain. The rain may change over to a wintry mix before ending
across the highest elevations before ending.
For the rest of Sunday through Monday, surface ridging will produce
dry and mild conditions over the region.
For Monday night through Wednesday, again the GFS and European model (ecmwf)
difference significantly in the development of cyclogenesis over the
mid-section of the nation. These differences are mainly due to the
phasing/interaction of the northern and southerly stream jets.
Either way, another round of rainfall is expected. If GFS evolution
does prevail, a line of convection is possible.
18z taf discussion.
Tri and tys are finally beginning to scatter out with cloud bases
rising. This will continue through the afternoon with VFR cigs
prevailing at all sites. Expect VFR to prevail tonight through
tomorrow morning too. Tri will have the lowest bases at broken
5000-7000 feet at times, but cigs should never fall back below
VFR. Patchy fog could occur between 08 and 13z for brief periods
at tys and tri, but not confident enough to include that in this
Preliminary point temps/pops...
Chattanooga Airport, Tennessee 40 66 49 63 54 / 0 0 30 90 90
Knoxville McGhee Tyson Airport, Tennessee 38 66 49 59 52 / 0 0 60 90 90
Oak Ridge, Tennessee 38 64 49 57 50 / 0 10 70 90 90
Tri Cities Airport, Tennessee 34 63 46 56 46 / 0 0 60 90 70